Screenshots from here on out will have to be taken from within the game itself. The official statement from Valve states:

“We ask that any images you upload to the ‘screenshot’ section of your store page should be screenshots that show your game. This means avoiding using concept art, pre-rendered cinematic stills, or images that contain awards, marketing copy, or written product descriptions.”

This information was first leaked by a member of the Facepunch forums who managed to get a screencap of letter from Valve themselves. This was then confirmed by Kotaku.

This is a huge step in the right direction for Valve and Steam. To have developers be accountable for the information that they show will make our job as media much easier. But also allow consumers to make accurate assessments on whether or not a game (that they discover by chance on Steam) is for them or not.